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[1] In 2000-2002, three rangeland and six irrigated sites were instrumented to assess the storage and transit time of chemicals in thick (15 to 50 m) unsaturated zones (UZ) in the High Plains. These processes are likely to influence relations between land use and groundwater quality, yet they have not been documented systematically in the High Plains. Land use and climate were important controls on the size of subsoil chloride, nitrate, and pesticide compound reservoirs. The reservoirs under irrigated cropland generally were larger than those under rangeland because more chemicals were applied to cropland than to rangeland. In some cases, chloride and nitrate reservoirs under rangeland were larger than those under cropland, presumably because of long-term evaporative concentration near the base of the root zone. Natural salts mobilized by irrigation return flow accounted for as much as 60 and 80% of the nitrate and chloride reservoirs, respectively, under some cropland, as indicated by detailed chemical profiles and . Advective chemical transit times in the UZ under cropland ranged from about 50 to 375 years, longer than any of the instrumented fields had been irrigated, yet agrichemicals were detected at the water table at four of the six sites. The data provide evidence for the existence of slow and fast paths for water movement in the UZ, with larger subsoil chemical reservoirs occurring in areas dominated by slow paths. Implications of these findings with respect to water quality in the aquifer are significant because they indicate that the amount of chemical mass reaching the aquifer could increase with time as chemicals that still reside under irrigated fields reach the water table.

Abstract.Groundwater and stream water data collected at three headwater catchments in the Neversink River watershed indicate that base flow is sustained by groundwater from two sources: a shallow flow system within the till and soil and a deep flow system within bedrock fractures and bedding planes that discharges as perennial springs. Data from eight wells finished near the till/bedrock interface indicate that saturated conditions are not maintained in the shallow flow system during most summers. In contrast, the discharge of a perennial spring remained constant during two summer rainstorms, providing evidence that the deep flow system is disconnected from the shallow flow system in summer.

This paper describes a direct comparison of a•parent ages derived from 3H/3He, chlorofluorocarbons (CC!3 F and CC12F2), and ø•Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficia! aquifers in late Cenozoic Atlantic Coastal Plain sediments of the Delmarva Peninsula, on the east coast of the United States. Most of the apparent tracer ages agree within 2 years of each other for recharge dates between 1965 and 1990. Discrepancies in apparent tracer ages usually can be explained by hydrological processes such as mixing in a discharge area. Recharge rate calculations based on apparent tracer age gradients at multilevel' well locations agree with previous recharge estimates. High recharge rates on the Delmarva Peninsula result in nearly complete dissolved-gas confinement in the groundwater. The remarkable agreement between the different tracer ages indicates negligible mixing of waters of different ages, insignificant dispersion, minimal gas loss to the atmosphere, and insignificant sorption-desorption processes at this location. 1. Introduction Quantification of groundwater flow requires information on flow velocities and directions, and recharge and discharge rates. The classical approach is to calculate groundwater flows from hydraulic head gradients, hydraulic conductivity, and porosity. Standard laboratory and field methods for determining hydraulic conductivity commonly give conflicting results with large errors because of heterogeneities in most aquifers. Laboratory and field methods often provide limited resolution of these heterogeneities. Most groundwater flow models are calibrated to reproduce the measured potentiometric surface. Because measurements of hydraulic head and other field data form the foundation of groundwater flow models, the model results are limited by the uncertainty of these parameters. Therefore any tracer that provides direct age information is a valuable tool for calibrating groundwater flow models. Groundwater contamination provides an unintentional "dye" experiment that can give insight into travel times along flow paths. However, often the source function of the contaminant(s) (i.e., concentration and date(s) of spill) is not known, adding uncertainty to flow calculations. Most contaminants are chemically reactive and undergo adsorptiondesorption, cation exchange, precipitation-dissolution, oxidation-reduction, or biological reactions within an aquifer. Most of these reaction rates are currently unknown. Thus even if the source of the contamination is known, large uncertainties due to nonconservative behavior exist for travel time calculations based on position of contaminant plumes. Such "dye" experiments can only be applicable on a local scale and, therefore, are not a universal dating method for use in natural systems. Transient tracers are geochemical trace substances that have source functions and/or known (radioactive) decay rates that change with time. Transient tracers provide time information that can be used to infer flo...

Concentrations of the chlorofluorocarbons (CFCs) CFC-11 and CFC-12 were determined in groundwater from coastal plain sediments of the Delmarva Peninsula. CFC-modeled ages were calculated independently for CFC-11 and CFC-12, and agreed to within 2-3 years in the majority of the waters. Recharge temperatures, determined from dissolved nitrogen and argon concentrations, varied from 9 -+ 2øC over most of the peninsula to 14 +_ 2øC at the southernmost tip of the peninsula in Virginia. The CFC-modeled ages were examined in relation to the known hydrogeologic environment, both on regional scales and in more intensively sampled local scale networks. The CFC-modeled recharge years and measured tritium concentrations were used to reconstruct a tritium input function that was compared to the modeled tritium plus 3He distribution. Most of the present distribution of tritium in Delmarva groundwater is consistent with low dispersivities. The results of the study strongly support the use of CFCs for dating shallow, aerobic groundwater. INTRODUCTION Chlorofluorocarbons (CFCs, Freons, (The use of brand names in this report is for identification purposes only and does not represent endorsement by the U.S. Geological Survey) chlorofluoromethanes, halomethanes, or halocarbons), specifically CFC-11 (CCI3F) and CFC-12 (CC12F2), are chemically stable, manmade compounds that have been manufactured since the !940s and 1930s, respectively, for use as propellants in aerosol cans, as foaming agents in plastics, and as refrigerants and solvents. They have received widespread attention in recent years because of their long atmospheric lifetimes, their contribution to ozone destruction in the lower stratosphere, and their contribution as atmospheric greenhouse gases. Growth curves of atmospheric concentrations of CFC-11 and CFC-12 have been reconstructed using production records and release data prior to 1977 and worldwide atmospheric measurements thereafter [Elkins et al., 1993; Busenberg et al., 1993]. Based on these growth curves and CFC solubility relations [Warner and Weiss, 1985], concentrations of CFCs found in groundwater have been used as indicators of the time since the infiltrating groundwater was recharged and isolated from the soil air assuming that the soil air is in equilibrium with the troposphere [Busenberg and Plumruer, 1992]. Results from previous studies confirm the utility of CFCs as dating tools and groundwater tracers [Randall and Schultz, 1976; Schultz et al.; Plummer et al., 1993a, b]. Busenberg and Plumruer [1992] introduced a sampling procedure for collection and storage of water samples in borosilicate glass ampules that are flame sealed in the field. CFCs remain stable in these ampules for many months and can be returned to the laboratory for analysis. Plumruer et al. [ 1993a] summarize the CFC dating method, evaluate effects of hydrodynamic dispersion on CFC dating, and compare CFC dating to dating with other environmental tracers. The present study uses CFC-11 and CFC-12 to date shallow groundwater from largely u...